1. Field of the Invention
The present invention relates to a supervisory system for a superconductive transformer and, more particularly, to a supervisory system which can detect quench of a plurality of windings constituted by superconductive lines in a short response time.
2. Description of the Related Art
In recent years, various superconductive lines with less AC loss have been developed. By constituting primary, secondary, . . . , Nth-order windings using such superconductive lines, attempts have been made to make a transformer compact and to reduce a copper loss.
In a so-called "superconductive transformer" in which primary, secondary, . . . , Nth-order windings are constituted by superconductive lines, when a current exceeding a critical current flows through the superconductive lines constituting the windings due to short-circuiting during an operation of the transformer, the corresponding windings are quenched. Once the quench occurs, the resistances of the windings are increased. For this reason, the superconductive transformer loses a function of an AC power transformer. Therefore, when the quench locally occurs, the superconductive transformer must be disconnected from a power source as soon as possible to prevent the quench of the remaining portion, and it must be waited until the quenched portion is recovered to a superconductive state. In order to perform such control, whether or not superconductive lines constituting the windings are quenched must be detected by any section.
As a method of detecting a quenched winding which is formed of the superconductive line and is used in an AC excitation state, a method of detecting the quench based on a phase relationship between a terminal voltage of the winding and a current flowing through the winding is known. This method utilizes the following phenomenon. That is, when the winding is excited with an AC voltage and when the winding is in a superconductive state, a phase difference of .pi./2 is accurately present between the voltage and the current. However, when the winding is quenched, the phase difference between the voltage and the current is decreased below .pi./2.
However, this method cannot be applied when a superconductive transformer is used. More specifically, in the superconductive transformer, a phase difference between a voltage and a current changes depending on types of load. When a load consists of only resistive components, there is no phase difference (=0); when a load consists of inductive components, the phase difference is approximate to .pi./2. For this reason, a phase difference caused by the quench cannot be discriminated. For these reasons, demand has arisen for a supervisory system which can detect quenched windings of the superconductive transformer in a short response time.
As described above, the conventional section cannot detect quenched windings of the superconductive transformer in principle.